The present invention relates to a sheet feeding apparatus used in copy machines, printers, printing machines, etc. and more specifically to a sheet feeding apparatus capable of separately feeding various types of sheets such as transparent sheets, scored papers, etc.
Copy machines, printers, and printing machines are equipped with a sheet feeding apparatus that feeds sheets one by one from a stack of copy paper by means of a sheet feeding roller. If this kind of sheet feeding apparatus feeds multiple sheets simultaneously, a paper jam may occur. If the feeding force is not strong enough, misfeeding may occur. To avoid these problems, there are methods to ensure that sheets are fed one by one. That is, increasing the friction coefficient between the sheet feeding roller and the sheet of paper so as to feed only the top sheet of the stack, thereby preventing misfeeding. An additional prevention against two or more sheets being fed simultaneously is a paper handling device, such as a paper handling roller, pat handler, or gripper, that does not allow sheets beneath the top sheet to advance so that only the sheet at the top of the paper stack is fed.
This method is effective only if ordinary copy paper is used. However, there has been an increased usage of copy machines and printers, which uses various different kinds of sheets such as transparent sheets, scored papers, and the like. Some types of paper adhere to one another when stacked that it is difficult for the above-mentioned sheet feeding apparatus to prevent two or more sheets from being fed simultaneously.
There is a proposed method that provides an air outlet on the lateral side to the sheet feeding direction of the paper stack, from which air is directed to several sheets at the top of the paper stack and flows through the sheets, thereby separating them. The sheets thus separated as mentioned above are fed one by one to the image forming section by means of a sheet feeding roller. Some printing machines employ a method of using suction to lift a sheet of paper located at the top of the stack and simultaneously transferring the sheet.
However, the suction method requires a compressor that can produce high suction, which results in large-scale equipment which increases power consumption and generates noise.
On the other hand, Japanese Publication Tokkaihei No. 04-23747 proposes, as shown in FIG. 5, a method in which air is blown from the lateral side to the feeding direction of the stacked sheets 100, and a lifted sheet is fed by means of a sheet feeding roller 2. This method also provides a hold-down member 3 at the rear end of the sheet 100 in the feeding direction so as to prevent sheets 100 from being lifted up excessively.
However, there is a problem with this method in that when the stacked sheets 100 are large, as shown in FIG. 5, several top sheets 100b of the stacked sheets 100 located in front of the air outlet 1 are lifted as a group. As a result, air does not flow through the sheets, thereby preventing individual sheets from being separated.
Furthermore, when a hold-down member 3 holds down the rear end of the stacked sheets 100, several top sheets 100b are inevitably lifted at the front end where a sheet feeding roller 2 is located. The sheet feeding roller 2 comes in contact with only a central part of the sheet; therefore, on both sides of the sheet feeding roller, several top sheets 100b of the stack are lifted, causing the following problems:
As stacked sheets are fed one by one from the top, the height of the uppermost surface of the paper stack is gradually lowered. This prevents the sheet feeding roller from coming in complete contact with the sheets, resulting in misfeeding. To prevent this, a sheet surface sensor is provided so that when the height of the uppermost surface of the paper stack is lowered, an elevating device lifts the stack of sheets 100 to maintain the stack at a certain height. This sheet surface sensor is generally located either on the left or right of the sheet feeding roller 2, that is the location where several top sheets 100b of the stacked sheets 100 are lifted. However, the position of the uppermost surface of the paper stack constantly changes in the vertical direction, consequently, it becomes impossible to reliably detect the position of the uppermost surface of the paper stack.
Moreover, when a hold-down member 3 is provided at the rear end of the stacked sheets 100, the location of the hold-down member 3 has to be changed according to different sizes of the sheets 100, thereby making the structure of the sheet feeding apparatus complicated.